vertexcodec: Optimize and cleanup NEON code
vertexcodec: Optimize and cleanup NEON code

This change gathers several small tweaks to make NEON code faster:

- Use dedicated lane load/store intrinsics instead of relying on the
optimizer to generate appropriate code; no perf change.

- Manually increment savep in the transpose loop instead of relying on
the compiler to remove multiplication; 0.38 GB/s -> 0.45 GB/s

zeux/meshoptimizer,zeux/meshoptimizer,zeux/meshoptimizer